For a decade, industrial demands on steel have become more and more severe because of the increasingly large mechanical stresses to which manufactured apparatuses are subjected and the reliability which is demanded of such apparatuses. The mechanical construction industry demands from steelmakers steels which have characteristics never before required, particularly as regards steel plates for welded structures.
This requires the steel technologists to solve numerous problems in order to reduce the risks of delamination and of anisotropy of steel plates. It is known that such delamination is due to inclusions of oxides and sulfides which have been greatly drawn out during rolling and which are present beneath a weld bead, parallel to the surface of the plate.
The steelmaker's efforts are thus centered on processes which can reduce inclusions of sulfides and oxides and the formation of flakes.
For this purpose, a high-quality metal has to be produced by:
reducing the sulfur content to less than five parts per thousand,
reducing the quantity of oxides and sulfides,
attempting to obtain inclusions of globular shape, uniformly distributed and pouring easily,
limiting the hydrogen content.
It has been established that the injection of alkaline earth metal (silico-calcium, silicon carbide) into steel contained in a ladle having a basic lining results in an extremely low oxygen content and a very considerable lowering of the sulfur content, which prevents formation of manganese sulfide (easily deformable and the cause of dangerous inclusions).
The alkaline earth metals reduce the proportion of metallic oxides SiO.sub.2, Al.sub.2 O.sub.3, giving rise to the presence of calcium silicate and aluminate having a low melting point, which favors their agglutination into large ferric droplets which pour easily and result in the production of a relatively clean metal of improved castability.
The large inclusions which are present (globular calcium aluminate) are not deformable at rolling temperatures, so that linearly drawn-out inclusions are not present, which give rise to delamination.
Several metallurgical processes have been advised for this purpose.